1. Field of the Invention
The present invention relates to a process for selectively depositing a refractory metal layer on a piece made from graphite, applicable in cases where such metal must be deposited on a limited surface of the graphite piece, as for example, in the manufacture of anodes for X-ray tubes.
2. Description of the Prior Art
Taking for example X-ray tubes having a rotary anode, they are currently called on to withstand high electric power, for varying operating times, to build up a charge. The power applied during this charge is intended to generate an electron flow, focussed on a small surface of the anode called the focal point, this surface becoming the source of X-rays. Rotation of the anode defines a focal ring formed by the surface of the rotating focal point.
The surface of this focal ring is generally formed by a metal or a metal compound, preferably refractory and with a high atomic number, such as tungsten or tantalum for example.
A small part of the energy received by the X-ray tube during the application of a charge is transformed into X-rays, the rest being dissipated in the form of heat, the removal of which from the anode can only take place by radiation. This is why it is of the greatest importance, with an anode formed from a graphite disk, for example, to limit the surface of the refractory metal at the surface of the focal ring, thus allowing a greater graphite surface to be obtained, which is capable of better heat radiation than the refractory metal.
The refractory metal surface forming the focal ring may be obtained by depositing a layer of the metal on the graphite disk; for that it is generally necessary to previously deposit a so-called intermediate layer over the whole surface of the disk, which allows more particularly a sufficient adhesion of the metal to the graphite to be obtained. Deposition of the refractory metal layer may then be achieved by different methods such as: gaseous phase chemical deposit, igneous electrolysis, plasma, vacuum deposit, etc.
Up to now, delimitation of the surface of the refractory metal deposit is obtained by using masks, these masks being disposed during the operations for depositing the refractory metal layer on the surfaces where such metal is undesirable.
This method of defining the refractory metal surfaces presents the following numerous drawbacks:
taking into account the environment to which the masks are exposed, during the operations for depositing the refractory metal layer, these masks are made from an expensive material, such as graphite, for example;
these masks require complex machining to give them, for example, the shape of half shells, which must be assembled together to protect peripheral surfaces of the disk from the refractory metal;
because of the addition of these masks, there is caused an increase of the total volume of the disk to be treated, which may lead to modification of the operating conditions during depositing of the refractory metal layer;
refractory metal is deposited on the masks themselves which leads, on the one hand, to an increase in the consumption of the metal and, on the other hand, to a variation in the size of the masks, detrimental to re-use thereof, since the limits of the protected zones are modified after each use.